Solid fuels represent a much greater percentage of the earth's known fuel reserves than oil or gas. It is therefore advantageous to develop more convenient, lower cost and more versatile methods of heating with a variety of solid fuels. The simplest, most direct method of heating with solid fuels is to make direct use of the efficient, heat radiating source that solid fuels become when they are very hot. Radiant heat is the quickest and most efficient means of transferring heat from solid fuel to non-gaseous heat absorbers. Maintaining high temperatures on emitting surfaces and better focusing of the radiant heat in the direction of utilization are key to efficient radiant heating. This can be understood better from the following expression for radiant heat energy exchange: ##EQU1## E is the radiant heat energy T2 is the temperature of the heat source
T1 is the temperature of the heat recipient PA1 A1 is the effective aperture area of the heat source PA1 E1 is the emissivity of the heat source PA1 d is the distance between the heat source and recipient PA1 KEA is the emissivity and aperture area of the heat recipient.
The combustion process, which is the source of heat energy, is a function of the fuel temperature and the availability of oxygen. When excessive demand for heat is placed on the combusting solid fuel, the heat transfer rate might exceed the supply of heat energy. This causes the fuel temperature to drop until an energy flow balance is achieved. This drop in fuel temperature can impair combustion efficiency or stop the combustion itself. The variability of solid fuels, particularly their moisture content, further complicates combustion. Solid fuel combustion units require more elaborate fire tending than other types of fuel burners. The fire tending includes continual insertion of fuel, poking and ash removal. Solid fuel combustion units also tend to have more air polluting emissions. The large variety of solid fuels, which include coals, peat, lignite, wood and various waste products contain various impurities and also have different degrees of combustibility. Many of the problem impurities that occur in solid fuels tend to burn off at lower temperatures. A combustion process where early combustion occurs in specified areas and where the emissions from this combustion are easily separated from the main flow of combustion supporting air, makes the processing of emissions easier to achieve.
It will become increasingly important in the future to have combusters that are flexible, burning a variety of fuels in a given combuster, for economic reasons. Also for both environmental and economic reasons, it will become increasingly attractive to burn sludge, garbage, tree bark, etc., converting these items into useful heat energy.
In the home, the fireplace, which has become largely ornamental, has potential as an auxiliary and emergency heating and cooking source. Improving the space heating efficiency of the woodburning fireplace, without destroying the mood created by an open fire, would be useful for many homes.